Hand held drills, both manual and power drills, require that the operator must hold the drill with the drill bit at precisely 90.degree. to the workpiece.
If the drill bit is not held true then the hole will deviate from its true course. This can happen even when a pilot hole is drilled out to a larger size.
Many different devices have been proposed for the purpose including sleeve-like devices designed to receive the drill bit and ensure that it is held normal to the workpiece. Obviously, such a sleeve must fit the shank of the drill bit accurately so that a number of different sleeves must be provided for different sizes of drill bits. In addition, such sleeve devices may make it difficult to center the drill on the punch or other marking on the workpiece.
It has been proposed in U.S. Pat. No. 3,436,155 to employ a metal panel having a reflective surface and having a series of different size holes through the panel to receive different sizes of drill bits. The shank of the drill bit is reflected on the surface of the panel and in this way it is possible visually to control the angle of the drill bit relative to the workpiece.
This proposal however was still subject to many of the disadvantages inherent in the sleeve type of drill guide. For example, it was necessary to provide a fairly large panel. The panel would have to be provided with a number of different holes one for each of the drill sizes for which the panel was designed. Obviously, in order to be effective, each of the holes would have to be spaced a considerable distance from the other holes so as to provide a sufficient reflective area around that particular hole so as to provide effective optical guidance for the drill bit. One of the disadvantages of the relatively large size of panel is that when working on many surfaces, holes will be required to be drilled close to an edge and it may be impossible to use the relatively large reflective metal panel in a confined area such as a corner or edge of a workpiece. Furthermore, if the panel is relatively thin it will soon become worn around the edges of the various holes and then become inaccurate and such a thin panel cannot be clamped in position on a workpiece. On the other hand, if the panel is made sufficiently thick to withstand substantial wear it will become excessively costly and cumbersome and such a thick panel has the effect of partially hiding the centre punch mark on the workpiece. Thus if the panel slips, the operator will not readily notice it and the hole will be off centre.
It will also be noted that as the panel was being handled, it would normally be carried in a tool kit where it would receive fairly rough treatment. It could be therefore anticipated that the reflective surface of the panel would very quickly become scarred and scratched and loose its effectiveness.
For all of these various reasons therefore the metal panel with a reflective surface was unsatisfactory for the purpose for which it was intended.